Radicals metal induced

The VO(OR)Cl2-induced ring-opening cycloaddition of diketene with styrenes leads to the 4,5-dihydofurans 66 [112], The oxidation of diketene with VO(OR)Cl2 generates a radical intermediate 67, which then adds to olefin, followed by one-electron oxidation to the corresponding cation, to give the dihydrofuran 66 (Scheme 2.52). A similar transformation was reported in the metal-induced radical cychzation of ethyl acetoacetate [113-115],... 

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

The use of y-ray induced radical pol5unerization proved to be a successful alternative for the radical co-polymer-ization of metal complexes with ligands containing acrylic C—C double bonds [100-102,129,130]. In particular, the palladium(II) complex cw-[PdCl2(ICPA)2] (1, Scheme 4) was co-polymerized in DMF solution with DMA and MBAA (cross-linker, 4% mol), with no degradation of the metal center [100,101]. 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

A review of the generation of aminyl radicals by photolysis and by metal-induced one-electron reductions of TV-chloroamines has appeared379. 

W. A. Nugent and T. V. RajanBabu, Transition metal-centered radicals in organic synthesis— Ti(lH)-induced cyclization of epoxyolefins, J. Am. Chem. Soc. 770 8561 (1988). 

A metal-induced one-electron reduction is frequently used to generate radical species. Termination of the radical reactions is due to a one-electron reduction process to give anions and therefore constitutes a non-chain process. As featured in Scheme 6.32, in many cases the multicomponent processes described here are a combination of radical and anionic bond-forming reactions. 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

As mentioned above, autoxidation studies124 indicate that reaction (104) is much faster than reaction (105) in hydrocarbon media. The kinetics of the reaction between manganous stearate and n-decyl hydroperoxide have been studied, and both metal-induced and radical-induced decomposition of hydroperoxide were observed. 

Aliphatic iodides, and especially secondary and tertiary representatives, are subject to hydride ehmination and are not generally useful substrates in transition metal catalysed coupling reactions. The last reaction in Scheme 14 [31], for instance, cannot be executed using current transition metal-based technology. In contrast, vinyl and aryl iodides, which are superb partners in many classical metal-induced couphng reactions, are very poor substrates in the present radical process because of the high energy of vinyl and aryl radicals. The two methods thus nicely complement each other. 

A radical species may also be generated by reduction of an electron-deficient compound and a classical entry to 4,4 -bipyridines is the reduction of a pyridine by sodium and subsequent rearomatization. Figure 8a illustrates the use of such a reduction in order to prepare the precursor of a sodium-ion molecular switch [30], A more general route, derived from the ancient copper-catalyzed Ullmann coupling, is the metal-induced dimerization of an aryl halide. The key step is a reductive elimination within the coordination sphere of the metal. A nickel(O) complex, in stoichiometric quantities, is usually selected for this purpose. Constable and Ward have used such a reaction to prepare a bis-terpyridine from an interesting synthon, which would have otherwise required a more specialized strategy with dedicated intermediates (Figure 8b) [31]. 

Although most of the thinkable principal substituents able to activate the cyclopropane core had been examined by the time this review was written, not all possible combinations of these functional groups had been tested. Yet multiactivated cyclopropanes should be a rewarding field displaying a particularly rich and useful chemistry as has been indicated by the behaviour of donor-acceptor-substituted cyclopropanes. These multiactivated cyclopropanes as well as free radical reactions and transition metal-induced processes deserve to attract more attention in future. 

Chevion. M. A site-specific mechanism for free radical induced biological damage the essential role of redox-active transition metals. Free Radic. Biol Med 5 2" -37 1988. 

Landoph JR (1999) Role of free radicals in metal-induced carcinogenesis. In Sigel A and Sigel H (eds.) Metal Ions in Biological Systems, Vol. 36, Interrelations between Free Radicals on Metal Ions in Life Processes, pp. 445-483. New York Dekker. 

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